Open pit protection design for a residential building in Zagreb
In the northeastern part of the city of Zagreb, the construction of a semi-buried residential building with three residential units is planned. Two residential units are to be constructed on the first floor and the third one on the second floor, while a garage is also planned for the underground level. The maximum dimensions of the building’s floor plan, which will incorporate an atrium measuring 4.0 m × 5.38 m, will be 22.75 m × 10.53 m. The total height of the building is designed to be 10.50 m. All the floors will be interconnected by a reinforced-concrete, two-flight staircase as well as an elevator.
Geotechnical investigation and testing
For the purpose of creating a temporary open pit protection design, engineering geological and geotechnical investigation and testing were carried out. The works consisted of the geological and geotechnical prospecting of a wider area, exploratory rotary drilling (two boreholes in total), the engineering and geological mapping of the terrain and the determination of drill cores, standard penetration testing (SPT), laboratory tests and data processing of previous investigation as well as correlation with the new data.
By processing the data from the geotechnical investigation and testing, it was determined that two geotechnical units exist at the location. These units, based on their physical and mechanical characteristics, serve as a cover (made ground and clay). Given the limited thickness of the made ground, the terrain at the location is characterized by a single geotechnical unit (GJ-1), namely clay.
Open pit protection
For the purpose of the construction of the future building, it is necessary to cut into the ground at a maximum depth of 4.70 m. Excavation is to be carried out in the cover, mostly in the clay layer, and because of the dimensions of the building plot and the proximity of the neighboring buildings, the excavation itself is divided into several zones.
Zone 1 is the southern border of the open pit and the building plot, next to which there are two existing neighboring buildings: a family home and the attached garage. Because of the proximity of the external walls of the existing and future buildings, the pile wall was chosen as the optimal solution in Zone 1. This type of protection is to be carried out in a width of approx. 30.0 m, while the remaining 8.0 m of the southern part of the pit next to the adjacent garage is to be dug vertically to a depth of approx. 2.50 m. The pile wall is to be made in a cascade, and the installation of geotextiles and one layer of Q-188 reinforcing mesh is planned to prevent material shedding between the micropiles. The pile wall is to be made in a cascade, and is to be connected by a reinforced-concrete capping beam with the following dimensions: H × V = 0.35 m × 0.40 m. The capping beam is also to be made in a cascade, heightwise following the lower elevation of the basement ceiling slab of the future building.
Zone 2 is the western and northwestern part of the open pit, which, because of the sufficient distance of the future building from the edge of the plot, is to be excavated at a slope of 1.5V:1H. In order to ensure the erosion stability of the slope, it is planned to place a construction PVC film on the face of the sloped excavation.
In Zone 3, the same type of protection is to be conducted as in Zone 1: a pile wall made of micropiles with a diameter D = 330.0 mm at an axial distance s = 0.60 m and with a length L = 6.0 m. The pile wall is to be conducted to the width of the neighboring family home on the south side and, as an additional form of protection, prop loads with a length L = 10.30 m are to be installed.
Prop loads, as additional protection, were chosen because of the height of the excavation and the impossibility of using anchors through the pile wall. A total of five prop loads are to be installed in the form of round steel tube profiles measuring 219.1 mm × 8.8 mm at an axial distance of 2.40 m, which are to be attached to the faces of the pile wall in Zone 1 and Zone 3.
Zone 4 is the northeastern part of the open pit, which is to be partially excavated at a slope of 1.5V:1H and to a length of approx. 17.0 m, while the remaining 8.0 m is to be excavated vertically. The slope of excavation is protected with construction PVC film.
Conclusion
During the excavation and protection of the open pit for the construction of a semi-buried residential building, it is necessary to conduct continuous geotechnical supervision and monitoring of the pit’s movement.
The measuring equipment to be used for monitoring comprises six geodetic measuring points, one inclinometer and one clinometer. Given the dimensions of the building plot and the proximity of existing structures, designing the protection for the open pit presents a challenge. However, the selected protection methods ensure its overall stability.
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